Straightedge Theory: Full of Holes?
When it comes to making wooden shop tools, I think that most of the modifications we make to them are to stop us from throwing the tools away by accident.
Many winding sticks are a right triangle in profile. Why? Mostly (I think) to prevent you from pitching them out with the garbage. Yes, the wider base makes them more stable, but not significantly so.
But when it comes to wooden straightedges, there are some other theories.
They go like this:
1. You make the top edge of the straightedge taper to its ends. This does three things: It means your straightedge will have only one “straight edge.” That’s a good thing because you have only one edge to maintain and won’t get two edges confused. And, the theory goes, it allows the straightedge to respond more rapidly to changes in humidity.
Wood loses and gains most of its moisture content through the end grain. So by exposing more of the end grain along the straightedge, the middle section is more likely to change in width at the simultaneously as the ends. With a straightedge with parallel edges, the ends will shrink and swell before the middle gets the moisture message.
And the bevels make you less likely to throw the straightedge in the burn pile.
2. You bore a hole or oval in the middle of the straightedge. This does three things: It makes the straightedge easier to carry around. It exposes more of the end grain in the middle of the tool to the atmosphere (see No. 1 above). And it makes you less likely to transform it into a sword for your 5-year-old Conan the Barbarian.
3. You lightly chamfer the long bevels and the rim of the hole. You add a finish. These details help prevent you from using the stick as a nine iron to drive your squirrel population into the neighboring county.
The big question for most woodworkers is about the moisture exchange. Do those construction details really help the straightedge stay true? I’ve made many straightedges using the above techniques and they seems to stay fairly straight. But what makes me think there is some evidence to support the idea is my winding sticks. They have parallel long edges. They have to, or they don’t work.
Right now, both my winding sticks are 5 thou fatter at the ends than at the middle. And I’ve been in Deep South humidity with them this summer.
So I think these construction details are worth incorporating into your straightedge – if only to keep you from using the tool to skewer an entire brisket and run down the street yelling “Beef parade!”
— Christopher Schwarz
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